1. The Field of the Invention
The present invention is related to optical methods and apparatus for the non-contact inspection and characterization of a surface. More particularly, the present invention is related to methods and apparatus for approximating the "spectral integrated scatter" function of a surface, thereby permitting the total integrated scatter of the surface over any desired spatial frequency limits to be determined.
2. Technical Background
The ability to accurately measure physical properties of a surface is important in a variety of applications. Such physical properties include roughness, texture, waviness, and information relating to the profile of the surface. The measure of such physical properties is generally referred to as "characterizing" a surface.
For example, in the field of computer hardware, it is preferable that computer hard disks be manufactured with a known roughness, generally referred to as "texture" by that industry. As a quality control measure, hard disk manufacturers desire a measurement device which would permit them to quickly and easily measure surface roughness as precisely as possible. Current technology trends are moving toward surface texture levels requiring surface measurement down to about the 10 Angstrom level. It would be preferable if surface roughness could be measured to within 1 Angstrom or less.
Other applications where precise roughness measurements are desirable include the computer chip wafer industry. In manufacturing chip wafers, it is desirable that the front surface of the wafer be as smooth as possible and that the back side of the wafer is finished to a known roughness.
Also, the optical industry, particularly mirror manufacturers, desires high-precision measurement devices to gauge the quality of the surfaces of their optics. Such optics are typically employed in imaging systems such as those utilized in telescopes and satellites.
Some surface characterization instruments operate by contacting the surface. A profilometer is an example of such a device. A profilometer operates by dragging a stylus across a surface. The stylus is physically connected to a recorder which traces the profile of the surface. Mathematical analysis of the profile may be conducted to determine physical properties of the surface.
For many applications, such contact-based instruments and methods are unacceptable because of the risk of contamination or other damage to the surface. Additionally, they are extremely slow and do not provide sufficient resolution to be effective for use in many applications. Thus, there exists a great need for noncontact surface characterization devices and methods.
Surface inspection devices based on optics have generally proved to be the most effective at noncontact surface characterization. Such optical devices typically operate by directing a beam of light at the surface and measuring the amount and direction of nonspecular light scattered off the surface. Through the analysis of such data, much information regarding the character of the surface can be ascertained.
One such noncontact, optical-based device is the scatterometer. To measure roughness, for example, the scatterometer measures the scatter intensity of the scattered light at every scatter angle in a selected plane. This information can then be used to generate the "power spectral density" function for that plane. The power spectral density function illustrates the distribution of the power scattered by each spatial frequency. The roughness of the surface can then be approximated by integrating the power spectral density function.
One disadvantage to the use of such scatterometers is that because the scatterometer measures only one plane of the scatter hemisphere, only a small portion of the total information about the surface is obtained. If the surface is isotropic, such methods are generally accurate. For isotropic surfaces, the total roughness is determined by performing three-dimensional integration on the power spectral density function. However, if the surface is nonisotropic, such as surfaces having a "lay" to them or randomly rough surfaces, a scatterometer may produce grossly inaccurate results.
One method for characterizing nonisotropic surfaces is to measure the scatter intensity at every point in the scatter hemisphere. The sample data can then be manually integrated to determine the roughness. Such a method can be performed with an "out-of-plane" scatterometer. This method is extremely time consuming and is therefore not practical for most applications which require rapid inspection and analysis.
In an attempt to make scatter measurement more efficient and versatile, it has been noted that plotting the power spectral density versus the spatial frequency on a log-log plot will generally result in a straight-line curve. Thus, by obtaining two representative points on this line, the curve can be approximated. By integrating this function over selected spatial frequency limits, surface roughness can be determined.
One difficulty with this process is that the power spectral density data is two dimensional; thus, the process only works well for isotropic surfaces. Additionally, because of the limitations on the physical size of the detector, the representative points used to generate the curve are close together. Hence, any noise in the data could substantially decrease the accuracy of the fit of the curve.
Measuring additional data points to improve the fit of the curve becomes difficult because of the complexity of the necessary instrumentation. Additionally, the math to include additional data points becomes unduly complicated. Also, the inclusion of more data points still does not account for nonisotropic variations in the surface. Thus, attempting to add additional data points to improve the curve fit is not viable for many applications.
The prior art method which is currently preferred for characterizing nonisotropic surfaces is the "total integrated scatter" method. According to this generally accepted method, an optical integrating device, such as a hollow sphere, generally referred to as an "integrating sphere," is placed over the surface of the sample. The integrating sphere has an input aperture through which a beam of light may be directed into the device. A sampling aperture on the other end of the sphere permits the light to be directed onto the surface and allows light scattered off the surface to enter the sphere. An output aperture is also configured into the sphere for permitting the reflected specular beam to exit the sphere. Thus, the light scattered off the surface remains within the sphere and its intensity can be measured with a detector. Advantageously, this method measures all of the scattered light regardless of variations in the surface.
Because the integrating sphere captures all of the scattered light, it performs a physical integration of the power spectral density function. Because the integration is performed directly by the sphere, some specific information about the power spectral density function is not obtained. For example, information regarding the slope of the function is not obtained.
Additionally, when using an integrating sphere, the limits of integration are set by the physical configuration of the sphere and cannot be changed without changing the physical configuration of the sphere. Also, because of physical limitations on the size and configuration of the sphere and on how the sphere may be positioned with respect to the sample, some limits of integration may not be obtainable.
Other problems associated with integrating spheres include the difficulty of preventing stray light from entering the sphere while containing all of the scattered light within the sphere. A principal source of stray light is the optics in the light source. Although the source optics focus the main beam, the optics also act as a scatter source.
By reducing the size of the input aperture of the sphere, much stray light can be blocked from entering the sphere. If, however, the size of the input aperture is too small, it will clip the main beam and cause diffraction of the main beam into the sphere, thereby introducing more stray light into the sphere. Thus, the input aperture must be sized larger than the main beam, thereby allowing some stray light to enter the sphere.
Another method of reducing stray light within the sphere is to increase the distance between the source optics and the input aperture. This decreases the effective size of the input aperture from the perspective of the source optics. Of course, making the instrument too large is not desirable. Thus, there are physical and practical limitations on the extent to which the distance between the source optics and the input aperture can be maximized.
The size of the output aperture also affects the amount of stray light contained within the sphere. The stray light introduced by the source optics is concentrated in the region surrounding the main beam. Thus, by increasing the size of the output aperture, much of this stray light will exit the sphere and therefore not be measured. However, much of the light scattered off the surface is also concentrated around the area of the main beam. Hence, increasing the size of the output aperture permits more scattered light to exit the sphere, thereby decreasing sensitivity of the instrument.
Additionally, the range of spatial frequencies over which the sphere integrates is determined in part by the size of the output aperture. If the size of the output aperture can be kept to a minimum, the range of spatial frequencies over which the sphere may integrate is increased.
For some applications, it is necessary to compare data obtained from different integrating devices. Because integrating devices frequently operate over different limits of integration (i.e., over different spatial frequency ranges), meaningful comparison of data obtained from different integrating devices is often impossible.
From the foregoing, it will be appreciated that it would be an advancement in the art to provide improved noncontact methods and apparatus for characterizing a surface. Indeed, it would be an advancement if such methods and apparatus could produce accurate results for nonisotropic surfaces.
It would be a further advancement in the art to provide an improved integrating sphere which would be small, convenient to use, and which could accomplish rapid inspection and analysis. It would also be an advancement in the art if such an integrating sphere could minimize stray light while maximizing the amount of scattered light measured by the sphere.
It would be a substantial improvement in the art if such an integrating sphere could be used to obtain the total integrated scatter for a surface over any desired spatial frequency limits, thereby permitting a variety of physical properties of the surface to be characterized and allowing comparison with data obtained from other integrating spheres.
Such methods and apparatus are disclosed and claimed herein.